1. Technical Field
The present invention relates generally to a semiconductor memory device, and more particularly to a randomizing and de-randomizing operation method of a semiconductor memory device.
2. Related Art
Semiconductor memory devices are generally divided into a volatile memory device and a nonvolatile memory device. While the volatile memory device loses stored data when power supply is interrupted or switched off, the nonvolatile memory device can retain stored data even in absence of power supply. Nonvolatile memory devices include various types of memory cell transistors. Nonvolatile memory devices are divided into a flash memory device, a ferroelectric RAM (FRAM), a magnetic RAM (MRAM), and a phase change RAM (PRAM).
Among nonvolatile memory devices, flash memory devices are generally divided into a NOR flash memory device and a NAND flash memory device, depending upon the configuration of memory cells and bit lines. The NOR flash memory device has a structure in which two or more memory cell transistors are connected in parallel to one bit line. Accordingly, the NOR flash memory device has an excellent characteristic in terms of random access time. Conversely, the NAND flash memory device has a structure in which two or more memory cell transistors are connected in series to one bit line. This structure is referred to as a cell string structure, and requires only one bit line contact per a cell string. Therefore, the NAND flash memory device has an excellent characteristic in terms of the degree of integration.
The semiconductor memory device such as a flash memory device may have various cell states such as ON cells and OFF cells depending upon a threshold voltage distribution. The ON cells may be erased cells, and the OFF cells may be programmed cells. The threshold voltage of a programmed memory cell may be changed by various factors. For example, the threshold voltage of the programmed memory cell may be changed due to program disturbance or coupling between adjacent memory cells. This will be described in detail below.
For instance, in a program operation, a change in the threshold voltage of a selected memory cell depending upon programmed data may cause changes in the threshold voltages of the adjacent memory cells. Also, in a read operation, cell current flowing through a selected memory cell may vary depending upon the threshold voltages of adjacent memory cells. In other words, the threshold voltage of a memory cell may be changed depending upon data programmed in a selected memory cell or a data pattern which means the threshold voltage distributions of adjacent memory cells.
The influence by the program disturbance or the coupling between memory cells may vary depending upon a data pattern. Therefore, in order to reduce variations in the threshold voltages of memory cells, a flash memory device may perform a randomizing operation for input data and perform a de-randomizing operation for data stored in memory cells.